In seismic exploration at sea, a plurality of seismic sensors are encased in a long tubular plastic cables which may extend for several miles. Depending on the respective type of seismic survey these cables are known as ocean bottom cable (OBC) or as streamers.
A streamer is towed by a seismic acquisition vessel through the water at a desired depth. A marine seismic source, such as an airgun, is used to generate acoustic waves. The acoustic waves are reflected from the earth layers below, to return to the surface of the water in the form of pressure waves. The pressure waves are detected by the pressure sensors and are converted to electrical signals.
A towed streamer comprises a plurality of pressure sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. This streamer may be divided into a number of separate sections or modules that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two-dimensional array of hydrophone elements. Data buses running through each of the modules in the streamer carry the signal from the hydrophone elements to the recording equipment (so-called acoustic data).
A hydrophone may produce electrical signals in response to variations of acoustic wave pressure across the hydrophone. Several hydrophones may be electrically coupled together to form an active section or group of an acoustic sensor array or streamer. Electrical signals from multiple hydrophones of an active section are typically combined to provide an average signal response and/or to increase the signal-to-noise ratio.
Recently, a new generation of streamers was introduced using so-called point receivers. In these streamers, signals can be recorded by individual hydrophones. Details of the new streamer design with the comparison to conventional streamers are described in the Summer 2001 edition of the Oilfield Review pages 16-31. For the purpose of the present invention it is important to note that in both, point receiver streamers and conventional streamer, hydrophones are arranged in essentially linear arrays in direction of the streamer.
The reflected sound waves not only return directly to the pressure sensors where they are first detected, but those same reflected sound waves are reflected a second time from the water surface and back to the pressure sensors. The surface-reflected sound waves of course, are delayed by an amount of time proportional to twice the depth of the pressure sensors and appear as secondary or “ghost” signals. Because the direct and surface-reflected sound waves arrive close together in time, they tend to interfere with one another or with other signals that propagate through the earth and share the same arrival time. It is therefore desirable to determine the direction of propagation of the sound waves so that the upward- and downward-propagating waves may be more readily separated during data processing.
In so-called dual sensor towed streamers, the streamer carries a combination of pressure sensors and velocity sensors. The pressure sensor is typically a hydrophone, and the motion or velocity sensors are geophones or accelerometers. In the U.S. Pat. No. 6,512,980 a streamer is described carrying pairs of pressure sensors and motion sensors combined with a third sensor, a noise reference sensor. The noise reference sensor is described as a variant of the prior art pressure sensor.
In practice, dual sensor towed streamers are difficult to use as the geophones deployed in the streamer generate signals proportional to vibrations of the streamer. Also, it is often not easy to correlate the respective outputs of hydrophones and geophones.
It is further known to position two individual hydrophones in a vertical array. It would of course then be relatively easy to identify the direction of propagation of the sonic waves from the measured difference in time that a particular wavelet arrives at the respective sensors that make up the vertical array, as described for example in U.S. Pat. No. 3,952,281. That method however requires two separate hydrophone cables. Since such cables cost about a half-million dollars each, this approach is hampered by the relative complexity of deployment and the high costs involved in duplicating the number of streamers in a survey.
In U.S. Pat. Nos. 4,547,869 and 4,692,907, it has been suggested to mount a substantially vertical array of sensors inside the same streamer, a few inches apart. But a seismic streamer cable twists and turns as it is towed through the water. This twisting and turning of a streamer makes it difficult to distinguish between the sensors in the vertical array. The '907 patent suggests the use of sensors with differential buoyancy inside liquid-filled chambers.
The '869 patent describes an acquisition system based on monomodal optical fibers using the difference in phase shifts due to the hydrostatic pressure of the optical signal of diametrically opposed pairs of fiber sensors as a means to identify the orientation of the fiber sensors. A similar streamer is described in EP 0175026 A1.
Outside the field of seismics, arrays of groups of hydrophones have been suggested for linear antennas (WO-03/019224 A1) and
In the light of the above, it is an object of this invention to provide an improved seismic acquisition system including arrays of hydrophones in a cable or a plurality of cables towed by a seismic vessel.